“The Biggest Bang” – Physicists Create Tunable Superconductivity in Twisted Graphene “Nanosandwich”

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Twisted Graphene Concept

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Structure might expose conditions required for high-temperature superconductivity.

When 2 sheets of graphene are stacked atop each other at simply the best angle, the layered structure morphs into a non-traditional superconductor, permitting electrical currents to go through without resistance or squandered energy.

This “magic-angle” change in bilayer graphene was observed for the very first time in 2018 in the group of Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. Since then, researchers have actually looked for other products that can be likewise twisted into superconductivity, in the emerging field of “twistronics.” For one of the most part, no other twisted product has actually displayed superconductivity aside from the initial twisted bilayer graphene, previously.

In a paper appearing in Nature, Jarillo-Herrero and his group report observing superconductivity in a sandwich of 3 graphene sheets, the middle layer of which is twisted at a brand-new angle with regard to the external layers. This brand-new trilayer setup displays superconductivity that is more robust than its bilayer equivalent.

The scientists can likewise tune the structure’s superconductivity by using and differing the strength of an external electrical field. By tuning the trilayer structure, the scientists had the ability to produce ultra-strongly paired superconductivity, an unique kind of electrical habits that has actually seldom been seen in any other product.

“It wasn’t clear if magic-angle bilayer graphene was an exceptional thing, but now we know it’s not alone; it has a cousin in the trilayer case,” Jarillo-Herrero states. “The discovery of this hypertunable superconductor extends the twistronics field into entirely new directions, with potential applications in quantum information and sensing technologies.”

His co-authors are lead author Jeong Min Park and Yuan Cao at MIT, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

Magic Angle Twisted Trilayer Graphene

This artist’s performance reveals magic-angle twisted trilayer graphene, made up of 3 honeycomb lattices. The securely bound electrons (yellow spheres linked by blue halos) show the brand-new structure’s highly paired superconducting state. Credit: Ella Maru Studio

A brand-new incredibly household

Shortly after Jarillo-Herrero and his coworkers found that superconductivity might be produced in twisted bilayer graphene, theorists proposed that the exact same phenomenon may be seen in 3 or more layers of graphene.

A sheet of graphene is an atom-thin layer of graphite, made completely of carbon atoms organized in a honeycomb lattice, like the thinnest, toughest chicken wire. The theorists proposed that if 3 sheets of graphene were stacked like a sandwich, with the middle layer turned by 1.56 degrees with regard to the external layers, the twisted setup would produce a type of balance that would motivate electrons in the product to pair and circulation without resistance — the trademark of superconductivity.

“We thought, why not, let’s give it a try and test this idea,” Jarillo-Herrero states.

Park and Cao crafted trilayer graphene structures by thoroughly slicing a single gossamer sheet of graphene into 3 areas and stacking each area on top of each other at the accurate angles forecasted by the theorists.

They made a number of trilayer structures, each determining a couple of micrometers throughout (about 1/100 the the size of a human hair), and 3 atoms high.

“Our structure is a nanosandwich,” Jarillo-Herrero states.

The group then connected electrodes to either end of the structures, and ran an electrical current through while determining the quantity of energy lost or dissipated in the product.

“We saw no energy dissipated, meaning it was a superconductor,” Jarillo-Herrero states. “We have to give credit to the theorists — they got the angle right.”

He includes that the specific reason for the structure’s superconductivity — whether due to its balance, as the theorists proposed, or not — stays to be seen, and is something that the scientists prepare to check in future experiments.

“For the moment we have a correlation, not a causation,” he states. “Now at least we have a path to possibly explore a large family of new superconductors based on this symmetry idea.”

“The biggest bang”

In exploring their brand-new trilayer structure, the group discovered they might manage its superconductivity in 2 methods. With their previous bilayer style, the scientists might tune its superconductivity by using an external gate voltage to alter the variety of electrons streaming through the product. As they called eviction voltage up and down, they determined the crucial temperature level at which the product stopped dissipating energy and ended up being superconductive. In by doing this, the group had the ability to tune bilayer graphene’s superconductivity on and off, comparable to a transistor.

The group utilized the exact same technique to tune trilayer graphene. They likewise found a 2nd method to manage the product’s superconductivity that has actually not been possible in bilayer graphene and other twisted structures. By utilizing an extra electrode, the scientists might use an electrical field to alter the circulation of electrons in between the structure’s 3 layers, without altering the structure’s general electron density.

“These two independent knobs now give us a lot of information about the conditions where superconductivity appears, which can provide insight into the key physics critical to the formation of such an unusual superconducting state,” Park states.

Using both techniques to tune the trilayer structure, the group observed superconductivity under a series of conditions, consisting of at a reasonably high crucial temperature level of 3 kelvins, even when the product had a low density of electrons. In contrast, aluminum, which is being checked out as a superconductor for quantum computing, has a much greater density of electrons and just ends up being superconductive at about 1 kelvin.

“We found magic-angle trilayer graphene can be the strongest coupled superconductor, meaning it superconducts at a relatively high temperature, given how few electrons it can have,” Jarillo-Herrero states. “It gives the biggest bang for your buck.”

The scientists prepare to produce twisted graphene structures with more than 3 layers to see whether such setups, with greater electron densities, can display superconductivity at greater temperature levels, even approaching space temperature level.

“If we could make these structures as they are now, at industrial scale, we could make superconducting bits for quantum computation, or cryogenic superconductive electronics, photodetectors, etc. We haven’t figured out how to make billions of these at a time,” Jarillo-Herrrero states.

“Our main goal is to figure out the fundamental nature of what underlies strongly coupled superconductivity,” Park states. “Trilayer graphene is not only the strongest-coupled superconductor ever found, but also the most tunable. With that tunability we can really explore superconductivity, everywhere in the phase space.”

Reference: “Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene” by Jeong Min Park, Yuan Cao, Kenji Watanabe, Takashi Taniguchi and Pablo Jarillo-Herrero, 1 February 2021, Nature.
DOI: 10.1038/s41586-021-03192-0

This research study was supported, in part, by the Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, and the Ramon Areces Foundation.